Dynamical aspects of the venus 4-day circulation

Evidence for the 4-day retrograde zonal circulation of the upper Venus atmosphere is summarized. The ‘moving flame’ phenomenon, convective instability to a mean shear and tidal forcing are discussed as possible dynamical explanations for the 4-day rotation. Tidal forcing seems feasible only if momentum diffusion is molecular in nature. Convective instability to a mean shear, although it can account for the magnitude of the 4-day circulation, must be supplemented by another mechanism, the ‘moving flame’ say, to explain the direction of the zonal motion. However, numerical computations indicate that the ‘moving flame’ by itself can account for both the magnitude and direction of the 4-day rotation. It appears that the stable stratification above altitudes of about 60 km is an essential factor in the ‘moving flame’ mechanism for generating the retrograde atmospheric rotation.